The 3rd Generation Partnership Project (3GPP) is starting to work on the standardization for 5G, i.e., 3GPP Release 14, in 2016 to make 5G a commercial reality in 2020. 5G is expected to be realized by continuous enhancement/evolution of LTE and LTE-Advanced and an innovative development by an introduction of a new 5G air-interface (i.e., a new Radio Access Technology (RAT)). The new RAT (i.e., New 5G RAT) supports, for example, frequency bands higher than the frequency bands (e.g., 6 GHz or lower) supported by the LTE/LTE-Advanced and its enhancement/evolution. For example, the new RAT supports centimeter-wave bands (10 GHz or higher) and millimeter-wave bands (30 GHz or higher).
Higher frequency can provide higher-rate communication. However, because of its frequency properties, coverage of the higher frequency is more local. Therefore, high frequencies are used to boost capacity and data rates in specific areas, while wide-area coverage is provided by lower current frequencies. That is, in order to ensure the stability of New 5G RAT communication in high frequency bands, tight integration or interworking between low and high frequencies (i.e., tight integration or interworking between LTE/LTE-Advanced and New 5G RAT) is required. A 5G supporting radio terminal (i.e., 5G User Equipment (UE)) is connected to both of a low frequency band cell and a high frequency band cell (i.e., a LTE/LTE-Advanced cell and a new 5G cell) by using Carrier Aggregation (CA) or Dual Connectivity (DC), or a modified technique thereof.
Non-Patent Literature 1 discloses user-plane and control-plane architectures to use both the LTE air interface (i.e., LTE RAT) and the new 5G air interface (i.e., New 5G RAT). In some implementations, a common Radio Resource Control (RRC) layer and a common Packet Data Convergence Protocol (PDCP) layer (or sublayer) are used. The common PDCP layer is connected to LTE lower layers and New 5G lower layers, and provides an upper layer with a transfer service of user plane data and control plane data through the LTE lower layers and the New 5G lower layers. The LTE lower layers include a Radio Link Control (RLC) layer, a Medium Access Control (MAC) layer, and a physical layer for the LTE-RAT. In a similar way, the New 5G lower layers include an RLC layer, a MAC layer, and a physical layer for the New 5G RAT.
The term “LTE” used in this specification includes enhancements of LTE and LTE-Advanced for 5G to provide tight interworking with the New 5G RAT, unless otherwise indicated. Such enhancements of LTE and LTE-Advanced are also referred to as LTE-Advanced Pro, LTE+, or enhanced LTE (eLTE). Further, the term “5G” or “New 5G” in this specification is used, for the sake of convenience, to indicate an air-interface (RAT) that is newly introduced for the fifth generation (5G) mobile communication systems, and nodes, cells, protocol layers, etc. related to this air-interface. The names of the newly introduced air interface (RAT), and nodes, cells, and protocol layers related thereto will be determined in the future as the standardization work progresses. For example, the LTE RAT may be referred to as Primary RAT (P-RAT or pRAT) or Master RAT. Meanwhile, the New 5G RAT may be referred to as Secondary RAT (S-RAT or sRAT).